Enhancing thrust



United States Patent 3,151,445 ENHANCING THRUST Wiliiam C. Bauman, Midland, Mich, assignor to The Dow Chemical tCompany, Midiand, Mich, a corporation of Delaware No Drawing. Filed Get. 14, 1959, Ser. No. 846,510 2 Claims. (Cl. 60-354) The present invention relates to rocket propulsion and more particularly to a method for enhancing thrust.

Frequently, much of the total energy created upon combustion of a propellant system is lost to the production of thrust due to absorption Within the combustion products of transitional heat associated with a high temperature product transformation such as a phase change or disproportionation. Such losses are especially characteristic of metal-containing reaction systems.

Another detrimental characteristic of some propellant systems is the high average molecular Weight of the combustion products. High molecular weight combustion products intrinsically limit the amount of thrust produced by a propellant system.

it is a principal object of the present invention to provide a method for effectively utilizing heats of transition normally associated with combustion product transformations, for the production of thrust. A further object of the present invention is to provide a method for improving the thrust of a propellant system by lowering the average molecular weight of the exhaust products. Other objects will become apparent hereinafter as the invention is more fully described.

The present invention involves a method for increasing thrust obtained from the combustion of a propellant system. Basically, the method comprises burning a propellant system within a combustion chamber of a rocket motor and incorporating into said chamber and into the presence of the burning propellant system an enhancing material to provide low molecular weight gas. Such low molecular weight gas is employed in the propellant combustion zone to utilize transitional heat associated with high temperature transformations in the combustion prod- V ucts for the production of thrust and also, to improve thrust by lowering the average molecular weight of the exhaust products.

As can be readily seen, an enhancing material has at least two functions. In the first instance, it provides a gaseous vehicle for absorbing and ultimately utilizing for the production of thrust in a rocket motor, heats of transition associated with high temperature transformations in the combustion products of propellant systems. The term transformation is inclusive of solid to gas, liquid to gas, and solid to liquid phase changes and disproportionation in the combustion products. High temperature transformation refers to combustion product changes occurring at or near the flame temperature of the primary propulsion reaction. In its second function, a limited amount of an enhancing material has a capacity, by virtue of its eifect in lowering the average molecular weight of the exhaust products, to cause a greater increase in the specific impulse of a system than is offset by any consequential lowering of the combustion chamber temperature due to the addition of enhancing materials at lower thermal energy levels.

Enhancing materials employed in the present invention must be capable of forming low molecular weight gaseous products having an average molecular weight significantly lower than the average molecular weight of the combustion reaction products, upon the application of heat at temperatures below the flame temperature of the primary propulsion reaction. Furthermore, it is most desirable that latent heats of phase changes in, and the heat capac 3,151,445 Patented Oct. 6, 1964 "ice ity of the resulting gaseous products of, enhancing materials be relatively small.

A suitable enhancing material may be an individual element having a low molecular weight or a composition of primarily low molecular Weight elements which composition dissociates into low molecular weight products upon the application of heat. A most effective enhancing material is one which has a positive heat of formation and therefore, upon dissociation, provides part of the heat required to raise its products to a desired combustion chambcr temperature. Enhancing materials operable in the present invention include compounds composed substantially of nitrogen and hydrogen such as ammonia, hydrazine, triazine, guanidine, diaminoguanidine, triaminoguanidine and the like. Also operable as enhancing materials are such low molecular weight substances as lithium, beryllium, boron, and the hydrides of these materials.

In order to improve the storage stability of the foregoing enhancing materials it may be desirable to employ them as a salt of nitric, nitrous, chloric or perchloric acids. In such instances, the acid radical advantageously supplies part of the oxidizer required for the propellant system. The remaining portion of the stabilized molecule constitutes an enhancing material which does not enter into the primary propulsion reaction.

The present invention can be most advantageously employed to enhance the thrust of propellant systems which upon combustion provide reaction products containing some proportion of solids or liquids which vaporize or disproportionate at or below the flame temperature of the reaction. Exemplary of such systems are those which employ metallic fuels since conventional oxidizers usually react with the metal components to form solid reaction products which may disproportionate, liquefy or vaporize at or below the flame temperature of the reaction.

Specific examples of metallic fuels operable in the present invention are beryllium, aluminum, magnesium, titanium, beryllium hydride, aluminum hydride, magnesium hydride, titanium hydride, and compositions containing these materials in combined form. Boron, its hydrides and compositions containing boron or its hydrides, such as diborane, pentaborane and decaborane, in combined form are also operable.

The oxidant employed can be any of the conventional oxidizers such as oxygen, fluorine and compositions containing these materials in combined form. Oxidizing nitrogen source materials are also eifective oxidizers for employment in the present invention. See my copending application Serial No. 846,509, filed October 14, 1959.

In carrying out the present invention, an enhancing material may be supplied to the propellant combustion zone along with either the fuel or the oxidizer of the system depending upon storage compatibility considerations. If it is not compatible with either of these propellant ingredients, the enhancing material can be independently injected into the reaction zone by separate means or maintained, as in the instance of solid propellants, as a distinct and separate phase in the presense of the other propellant ingredients. Enhancing materials are used in an amount sufficient to maintain the combustion chamber temperature below that at which high temperature transformations occur in the reaction products and in an amount correlative with an optimum improvement in thrust resulting from the lowering of the average molecular weight of the reaction products.

Representative of the present invention is the aluminum-ammonium nitrate-hydrazine combined reaction system shown below as a combination of (l) the aluminumammonium nitrate primary combustion reaction and (2) the hydrazine dissociation reaction which provides low .9 molecular weight gaseous products. In this equation, (X) is the number of moles and hydrazine added.

(1 2A1 NH,N A1203 N2 2 2A1 NH4NO3 (X) NzH-i 12 3 )(Nz) 2X)H2 By adding hydrazine, which has a dissociation flame temperature of about 1400" Kelvin, the specific impulse (a theoretical measure of potential thrust) of the combined reaction system is increased over a limited range of hydrazine addition as is shown in the following table:

In computing the specific impulse, for the above reaction system, for various amounts of hydrazine added and also for the other reaction system shown below, the reaction products are assumed to be ideal gases and the expansion through a rocket nozzle is assumed to be adiabatic. Further, it is assumed that the enhancing materials the utilization of transitional heat normally associated with transformations within the reaction products of the primary reaction system such as disproportionation of the A1 0 to lower oxides. An optimum efiect was achieved when the positive effects as described above were offset by the negative effects of a lower chamber temperature (T resulting from the addition of enhancing material at a lower thermal energy level that the primary reaction products. In the foregoing reaction system, the chamber temperature (T was reduced from approximately 4118 Kelvin which was the flame temperature of the primary reaction of aluminum and ammonium nitrate to about 2916 Kelvin at the optimum point.

When available, the thermo values employed in such thermodynamic calculations were those published in Circular 500 of the National Bureau of Standards.

The following equations and data show relative specific impulses achievable in other reaction systems through the use of hydrazine (with the exception of (1) (E) which illustrates the use of hydrogen) as an enhancing material to reduce the average molecular weight or" the exhausting products and to utilize transitional heat associated with transformations in the reaction products at or near the flame temperature of the reaction. Reaction equations preceded by (P) indicate the primary reaction system and those preceded by (E) indicate the enhanced reaction system.

dissociate to provide free hydrogen and nitrogen and that the products attain frozen equilibrium flow; i.e., there is complete heat exchange within the rocket motor. Corrections are incorporated for solid reaction products. The equation employed to compute the specific impulse is as follows:

()ther enhancing materials which may be substituted that for hydrazine with the achievement of similar results are helium, lithium, beryllium, boron, lithium hydride, boron hydride, beryllium hydride, ammonia, guanidine, diaminoguanidine, triaminoguanidine, triazine, S-amino tetrazole and the like.

I (specific impulse) wherein T =chamber temperature in degrees Kelvin, in the exhaust products, M =the average gram molecular M =the average gram molecular weight of the gases weight of the solids in the exhaust, N the number of moles of solids per mole of gas formed, C :the heat capacity of the solids in calories per gram mole per degree Kelvin, C =the heat capacity of the gaseous products at constant pressure in calories per gram mole per degree Kelvin, C =the heat capacity of the gaseous products at :onstant volume in calories per gram mole per degree Kelvin, P =the exhaust pressure (assumed to be 14.7 ).s.i.a.), P -chamber pressure (assumed to be 1000 a.s.i.a.), R=the universal gas constant and .2 0, By substituting appropriate values for the variables a the foregoing equation, various specific impulse values f the aluminum-ammonium nitrate-hydrazine reaction ystem were obtained for certain values of (X). This paricular system reached an optimum specific impulse when X) equalled 2.65. The addition of the enhancing ma- :rial hydrazine resulted in the lowering of the average iolecular Weight of the reaction products and also in Various modifications may be made in the present invention without departing from the spirit or scope thereof and it should be understood that the invention is limited only as defined in the claims.

I claim:

1. A method for increasing the thrust obtained from burning a primary propellant system in the combustion zone of a rocket motor which includes introducing into the combustion zone of a rocket motor a propellant system comprising a fuel and a self-contained oxidizer for said fuel, said propellant system upon combustion providing reaction products some proportion of which undergo at about the flame temperature of the combustion reaction of said propellant system phase transformation having transitional heats associated therewith and said propellant system being further characterized in that the fuel contains a member selected from the group consisting of beryllium, boron, aluminum, magnesium, titanium, diborane, pentaborane, decaborane, aluminum hydride, beryllium hydride, magnesium hydride and titanium hydride and the oxidizer is a member selected from the group consisting of oxygen, fluorine, oxidizing compositions containing these materials in combined form and oxidizing nitrogen source materials, and burning said 5 propellant system within the combustion chamber of said rocket motor the improvement which comprises;

(a) incorporating into said combustion chamber in the presence of said burning propellant system an enhancing material comprised substantially of nitrogen and hydrogen, said enhancing material being capable of absorbing and utilizing said transitional heats associated With the phase transformations of said propellant system combustion products to provide low molecular weight gaseous products thereby applying to said enhancing material said transitional heats at a temperature below the flame temperature of said propellant system and thereby dissociating said enhancing material and providing low molecular Weight gaseous products, said gaseous products having an average molecular weight significantly lower than the average molecular weight of the combustion products of said propellant system, and

(b) ejecting from said rocket motor the mixture of said combustion products from said propellant system and the low molecular weight gases from dissociation of said enhancing material, said mixture realized fr om said propellant system.

2. The process as defined in claim 1 wherein the enhancing material is selected from the group consisting of ammonia,

hydrazine, triazine, guanidine, diaminoguanidine, triaminoguanidine, S-amino-tetrazole and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS Alford et a1. May 1, 1951 Kolfenbach et a1. Feb. 26, 1957 Wasserbach et a1. Mar. 22, 1960 Stengel Apr. 11,, 1961 Johnson et al. Aug. 21, 1962 OTHER REFERENCES Penner: Journal of Chemical Education, January 1952,

Harvey: SAE Journal, vol. 65, August 1957, pp. 17-20. 

1. A METHOD FOR INCREASING THE THRUST OBTAINED FROM BURNING A PRIMARY PROPELLANT SYSTEM IN THE COMBUSTION ZONE OF A ROCKET MOTOR WHICH INCLUDES INTRODUCING INTO THE COMBUSTION ZONE OF A ROCKET MOTOR A PROPELLANT SYSTEM COMPRISING A FUEL AND A SELF-CONTAINED OXIDIZER FOR SAID FUEL, SAID PROPELLANT SYSTEM UPON COMBUSTION PROVIDING REACTION PRODUCTS SOME PROPORTION OF WHICH UNDERGO AT ABOUT THE FLAME TEMPERATURE OF THE COMBUSTION REACTION OF SAID PROPELLANT SYSTEM PHASE TRANSFORMATION HAVING TRANSITIONAL HEATS ASSOCIATED THEREWITH AND SAID PROPELLANT SYSTSEM BEING FURTHER CHARACTERIZED IN THAT THE FUEL CONTAINS A MEMBER SELECTED FROM THE GROUP CONSISTING OF BERYLLIUM, BORON, ALUMINUM, MAGNESIUM, TITANIUM, DIBORANE, PENTABORANE, DECABORANE, ALUMINUM HYDRIDE, BERYLLIUM HYDRIDE, MAGNESIUM HYDRIDE AND TITANIUM HYDRIDE AND THE OXIDIZER IS A MEMBER SELECTED FROM THE GROUP CONSISTING OF OXYGEN, FLUORINE, OXIDIZING COMPOSITIONS CONTAINING THESE MATERIALS IN COMBINED FORM AND OXIDIZING NITROGEN SOURCE MATERIALS, AND BURNING SAID PROPELLANT SYSTEM WITHIN THE COMBUSTION CHAMBER OF SAID ROCKET MOTOR THE IMPROVEMENT WHICH COMPRISES; (A) INCORPORATING INTO SAID COMBUSTION CHAMBER IN THE PRESENCE OF SAID BURNING PROPELLANT SYSTEM AN ENHANCING MATERIAL COMPRISED SUBSTANTIALLY OF NITROGEN AND HYDROGEN, SAID ENHANCING MATERIAL BEING CAPABLE OF ABSORBING AND UTILIZING SAID TRANSITIONAL HEATS ASSOCIATED WITH THE PHASE TRANSFORMATIONS OF SAID PROPELLANT SYSTEM COMBUSTION PRODUCTS TO PROVIDE LOW MOLECULAR WEIGHT GASEOUS PRODUCTS THEREBY APPLYING TO SAID ENHANCING MATERIAL SAID TRANSITIONAL HEATS AT A TEMPERATURE BELOW THE FLAME TEMPERATURE OF SAID PROPELLANT SYSTEM AND THEREBY DISSOCIATING SAID ENHANCING MATERIAL AND PROVIDING LOW MOLECULAR WEIGHT GASEOUS PRODUCTS, SAID GASEOUS PRODUCTS HAVING AN AVERAGE MOLECULAR WEIGHT SIGNIFICANTLY LOWER THAN THE AVERAGE MOLECULAR WEIGHT OF THE COMBUSTION PRODUCTS OF SAID PROPELLANT SYSTEM, AND (B) EJECTING FROM SAID ROCKET MOTOR THE MIXTURE OF SAID COMBUSTION PRODUCTS FROM SAID PROPELLANT SYSTEM AND THE LOW MOLECULAR WEIGHT GASES FROM DISSOCIATION OF SAID ENHANCING MATERIAL, SAID MIXTURE HAVING AN AVERAGE MOLECULAR WEIGHT LOWER THAN THAT OF THE COMBUSTION PRODUCTS OF SAID PROPELLANT SYSTEM THEREBY INCREASING THE THRUST OVER THAT NORMALLY REALIZED FROM SAID PROPELLANT SYSTEM. 